How to Spot a Failing Brake Booster

By Mia Bevacqua, April 25, 2018

The harder you hit the brake pedal, the quicker your car stops — but how does that pressure halt a 2,000-pound (or heavier) car so fast? The answer lies, in part, with the brake booster. Let's look at how it works, or you can jump ahead to symptoms of a bad one and learn what to do about it.

The brake booster is located between the brake pedal and your braking system’s master cylinder. When you push the brake pedal, the booster multiplies that force and transmits it to the master cylinder. The master cylinder then transfers pressurized brake fluid to the brakes through a series of lines. This pressurized fluid causes the brake caliper or wheel cylinder (depending on your system) to directly apply the brakes.

There are three common types of brake boosters: vacuum, hydro-boost and electrohydraulic. Vacuum is the most common overall. Hydro-boost is frequently found on diesel vehicles, and relies on the car’s power steering system. Electrohydraulic is most commonly seen on hybrid cars.

If you’re experiencing braking problems of any kind, it’s not safe to drive your car. Have it towed to a mechanic, who can diagnose and fix the problem.

Get it diagnosed by a professional

Signs of a failing brake booster

A faulty brake booster can cause a number of problems. These are some of the most common.

Problems associated with all three types of boosters:

  • Hard brake pedal: A problem with any type of brake booster can result in a pedal that’s difficult to depress.
  • Greater stopping distance: A failed booster reduces your braking power, which means it’ll take longer to come to a stop.

Problems associated with a vacuum booster:

  • Your check engine light is on, and your engine acts up: A leaking vacuum-style booster can rob the engine of manifold vacuum. This can cause the check engine light to turn on. The engine will likely run rough as well, and may even stall when you brake.
  • Hissing noise: A leaking vacuum booster can cause a hissing noise.

Problems associated with a hydro-boost booster:

  • Fluid leaks: Power steering fluid leaks can be caused by a faulty hydro-boost booster. If the leak is severe enough, you may notice that steering at low speeds becomes more difficult.
  • Grabby brakes: An internal hydro-boost failure can make the car feel like it’s jerking when you brake.  

Problems associated with an electrohydraulic booster:

  • Illuminated ABS or traction control lights: A computer monitors the operation of the electrohydraulic system. A booster problem can cause it to turn on the anti-lock braking (ABS) or traction control warning lights.
  • Fluid leaks: Brake fluid leaks can be caused by a faulty electrohydraulic booster.
  • Grabby brakes: An internal electrohydraulic booster failure can make the car feel like it’s jerking when you brake. 

» MORE: Get an estimate for a brake booster replacement

How to fix the problem

Diagnosing a vacuum or hydro-boost system is typically simple. A technician may only need to feel the brake pedal with the engine running to tell if the booster is at fault. 

The booster will then be tested manually. For vacuum boost systems, this means sucking the air from the booster, and observing pressure drop. For hydraulic systems, the hydraulic pressure going into the booster must be measured.

If it’s faulty, the brake booster must be disconnected from the vehicle by removing the master cylinder, brake booster vacuum or hydraulic lines, and finally the brake booster itself. A new one can then be installed.

This is a job best left to your mechanic, especially in the case of complex, electrohydraulic units. After replacement, both hydro-boost and electrohydraulic systems will need to be bled of air, and the brakes will need to be bled at all four wheels, as well.


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